1. Technical Field
The present invention relates to an image display device or a device for displaying images, and an image display screen thereof. In particular, the present invention relates to a laser image display device for forming an image using a laser light source as a light source, and a laser image display screen used with the laser image display device.
2. Description of the Related Art
Image display devices using various methods are currently being used widely. One of the image display methods used in such various image display devices is a projection display method in which light from a modulated light source is projected onto a screen and an image is displayed. Conventionally, the light source employed in the image display device using such method is a lamp light source. However, the lamp light source has problems in a short lifetime, a limited color reproduction region, and low light use efficiency.
In order to solve the problems in the lamp light source, a laser light source is recently being tested for use as a light source of the projection display. In this application, the image display device using the laser light source as the light source is referred to as the laser image display device, and similarly, the screen for displaying the image using the laser light source as the light source is referred to as the laser image display screen. The laser light source of the laser image display device has a longer lifetime than the lamp light source, and is easy to enhance the light use efficiency due to a strong directivity of the laser light. Furthermore, the laser light emitted by the laser light source excels in monochromaticity, enlarges the color reproduction region compared to the lamp light source, and enables a colorful image display.
However, the laser image display device has a problem of a speckle noise. The speckle noise is a noise caused by a high coherency of the laser light used in the image display. Such problem has not existed in the image display device that uses the lamp light source. When the laser light having high coherency is scattered on the screen and reaches an observer, the scattered laser light interfere with each other and the observer recognizes a microscopic uneven noise. Various techniques related to the laser image screen and the laser image display device for reducing the speckle noise have been proposed.
Japanese Laid-Open Patent Publication No. 55-65940 discloses a method of removing the speckle noise by vibrating the laser image display screen. However, a driving unit for vibrating the laser image display screen must be large when adopting such method for the large laser image display screen, and thus the volume of the driving unit and the power consumption increase.
Japanese Laid-Open Patent Publication No. 2003-98601 discloses a laser image display screen that may be used in a rear projection type laser image display device of projecting the image from a rear surface of the laser image display screen which is a side opposite to the observer and providing the image to the observer by transmitted laser light. The laser image display screen disclosed in JP 2003-98601 reduces the speckle noise with using a configuration which includes two types of light diffusion plate.
FIG. 1 is a schematic view of the laser image display screen disclosed in JP 2003-98601. The laser image display screen includes first and second diffusion plates 28a and 29b. This figure is a view describing a light path length difference provided by the laser image display screen with respect to two laser beams 25 and 27 that enter the first diffusion plate 29a at the same angle, transmit therethrough, and reach one region of the retina considered the same in recognition of an observer V. The laser light entering the first light diffusion layer 29a is diffused by the first diffusion plate 29a and propagated in various directions. At least a part of the laser light having exited the first light diffusion layer 29a is further entered to the second light diffusion layer 29b, diffused, and exits the layer and reaches the observer V. This screen is a laser image display screen for the rear projection type laser image display device that aims to reduce the speckle noise by using two diffusion plates 29a and 29b having different diffusion effects.
When using such screen for the laser image display screen, an infinite number of laser beams that diffuse in various directions exit the screen. The infinite number of laser beams includes the laser beams 25 and 27. The laser beams 25 and 27 enter the first diffusion plate 29a at the same incident angle from a right side of the figure. The laser beam 25 exits the first diffusion plate 29a at an angle of θ with respect to the main surfaces of the diffusion plates 29a and 29b due to a diffusion effect of the first diffusion plate 29a, and then enters the second diffusion plate 29b, at where the path is returned to the same direction as the incident angle with respect to the first diffusion plate 29a due to a diffusion effect of the second diffusion plate 29b, and reaches the eye of the observer V. On the other hand, the laser beam 27 enters the first diffusion plate 29 at the same incident angle as the laser beam 25, transmits the first and second diffusion plates 29a and 29b through the path of the same direction as that of the path when the laser beam 27 entered the plate, exits the plate and reaches the eye of the observer V.
The light path length difference of the laser beam 25 and the laser beam 27 is approximated asT((1/cos θ)−1)where T is a distance between the first diffusion plate 29a and the second diffusion plate 29b. θ is the diffusion angle of the first diffusion layer 29a. As θ approaches zero, the laser beams 25 and 27 strongly interfere with each other at the retina of the observer V. In this case, the light path length difference gradually approaches zero. Thus, the speckle noise that may be recognized by the observer V still has strong intensity.
In JP 2003-98601, no description related to problems such as a multiple reflection and the like of the laser light by the two types of light diffusion plates which is assumed to be arisen is made.
It is impossible to use the laser image display screen disclosed in JP 2003-98601 for a front projection type laser image display device, which projects the image from the same side as the observer, that is, a front surface of the laser image display screen and provides the image to the observer by reflected laser light.
The method disclosed in JP 55-65940 is applicable to the front projection type laser image display device. However, the method disclosed in JP 55-65940 is difficult to implement on the large laser image display screen for displaying large images, as mentioned above.